This invention relates to construction and control of communication networks that carry packetized data, e.g., Internet-protocol-based (IP-based) traffic, and in particular to network architectures that have large freedom in network topology and simplified network extensions with a lightweight but robust protocol. Applicant""s invention is applicable to wired and wireless networks handling packet data traffic, including cellular radio networks handling voice traffic.
It is generally expected that the importance of packet data networks, and in particular IP-based networks, will continue growing. Although a large proportion of current IP-based traffic is xe2x80x9cbest effortxe2x80x9d, in the sense that the network does not guarantee any minimum level of bandwidth or quality of service to a user, the proportion requiring some guaranteed level of quality will only increase in the future. Such guarantees may eventually be provided by the currently evolving differentiated services concept, which is described in D. Clark and J. Wroclawski, xe2x80x9cAn Approach to Service Allocation in the Internetxe2x80x9d, http://ietf.org/internet-drafts/draft-clark-diff-svc-alloc-00.txt, Internet Engineering Task Force (July 1997); K. Nichols, V. Jacobson, and L. Zhang, xe2x80x9cA Two-Bit Differentiated Services Architecture for the Internetxe2x80x9d, http://ietf.org/internet-drafts/draft-nichols-diff-svc-arch-00.txt, Internet Engineering Task Force (November 1997); and D. Clark and W. Fang, xe2x80x9cExplicit Allocation of Best Effort Packet Delivery Servicexe2x80x9d, http://diffserv.lcs.mit.edu/Papers/exp-alloc-ddc-wf.pdf (November 1997). The demand to access IP-based and other packet data networks from wireless terminals such as those operating in current cellular radio telephone networks will also increase.
Packet data networks are designed and based on industry-wide data standards such as the open system interface (OSI) model or the transmission control protocol/Internet protocol (TCP/IP) stack. In the communications industry, Layer 1 may be called the physical layer, and the Layer 1 protocol defines the parameters of the physical communications channel, e.g., frequency spacing of a radio carrier, modulation characteristics, etc. Layer 2 is called the data link, or hardware interface, layer, and the Layer 2 protocol defines the techniques necessary for the accurate transmission of information within the constraints of the physical channel, e.g., error correction and detection, etc. Layer 3 may be called the network, or resource control, layer, and the Layer 3 protocol defines the procedures for reception and processing of information transmitted over the physical channel. The functionality of a Layer 2 protocol includes the delimiting, or framing, of Layer 3 messages sent between communicating Layer 3 peer entities. In cellular radio telephone systems, an air interface protocol, such as TIA/EIA/IS-136 and TIA/EIA/IS-95 published by the Telecommunications Industry Association and Electronic Industries Association, is a combined Layer 1, 2, and 3 protocol that specifies how remote stations like cellular telephones communicate with base stations and a mobile services switching center (MSC).
These standards have been developed, whether formally or de facto, for many years, and the applications that use these protocols are readily available. The main objective of standards-based networks is to achieve interconnectivity with other networks. The Internet is today""s most obvious example of such a standards-based network in pursuit of this goal. One approach to packet-based wireless communication is cellular digital packet data (CDPD), aspects of which are described in U.S. Pat. No. 5,729,531 and U.S. Pat. No. 5,768,267, both to Raith et al., and U.S. Pat. No. 5,751,731 and U.S. Pat. No. 5,757,813, both to Raith; in allowed U.S. patent applications Ser. Nos. 08/544,492 and 08/544,589, both by Raith et al., and Ser. Nos. 08/544,837 through Ser. No. 08/544,839; and in U.S. patent applications Ser. Nos. 08/544,493 and Ser. No. 08/544,835, both by Raith, Ser. No. 08/544,836 by Bilsstrom et al., and Ser. Nos. 08/544,841 and Ser. No. 544,843, both by Raith et al. These patents, allowed applications, and applications are expressly incorporated here by reference.
The bottleneck of IP-based communication with a wireless terminal is the air interface, which may be shared among wireless terminals by using a random access protocol, possibly with some soft-state-based priority for continuous transmissions, or by using a protocol as in conventional cellular radiotelephony that allocates a wireless channel to a terminal on a first-come-first-served basis. For packet data traffic like IP-based traffic, a random access (connection-less) protocol may be more appropriate than a channel allocation (connection-oriented) protocol. Such IP-based information flows originated or terminated by a wireless terminal typically have lower bit rates than the average wired IP flow. This limitation is especially felt when a base station""s service area is large because the air interface capacity decreases with increasing cell size, as described in D. F. Bjornland et al., xe2x80x9cUMTSxe2x80x94The Universal Mobile Telecommunications Systemxe2x80x9d, Telektronikk (August 1996); IEEE Personal Communication vol. 4, no. 4 (August 1997); and E. Nikula et al., xe2x80x9cFRAMES Multiple Access for UMTS and IMT-2000xe2x80x9d, IEEE Personal Communications vol. 5, no. 2 (April 1998).
Wireless users may be willing to accept lower bit rates than wired users due to the air interface bottleneck, but wireless users will expect to use the same hardware and software as wired users. Thus, the wired part of the network should be unaware of the mobility of the users, except possibly for minor modifications. Although these features are not strict requirements, a communication system having these features can be expected to be more widely and easily accepted.
Although it is important, air interface bandwidth efficiency may not be the primary requirement for a cellular access network serving IP-based communication. Applicant believes that low equipment cost, robustness, scalability, low maintenance, and easy system establishment and expansion (growth), which are features typical of IP-based networks, will be more important to cellular network operators. Scalability is a feature that makes an architecture or technique applicable to large and small networks; expansion or extension are the actions necessary for making a network larger (e.g., plug in new nodes, etc.). These features are distinct, and both are desirable.
Another problem with cellular systems is that they must keep track of the movements of the served mobile users so that the users can be found when they have incoming calls/data. Usually the system is divided into a large number of cells, making it inefficient to maintain a central database that knows the exact location of each user""s terminal because frequent movements generate a large load of movement-indications sent to this central database. On the other hand, such exact location information is desirable because the more precise the location information is, the faster and less wastefully a remote terminal can be found for an incoming call. Network actions taken when a mobile station has an incoming call and has to be searched for (because its exact location is unknown) is usually called paging. The more exact the location information is, the fewer the cells that must be paged for the mobile, which means less load.
In most cellular systems (e.g., systems operating according the GSM standard), an intermediate solution is used. A central database has some information on the location of each remote terminal, but this information is not exact. The service area of the system is divided into location areas, each of which contains a number of cells. The central location database knows in which location area each remote terminal is, but not in which of the location area""s cells it is. When a remote terminal has an incoming call, the terminal is paged in all the cells of the known location area. The optimal point in the trade-off is achieved by selecting the proper size of the location areas.
This solution largely relies on a central database that may be duplicated for reliability but is still a point of failure. Due to the centralized approach, this solution also has scalability problems since location update messages sent to the central database impose a load on the network that gets more and more significant as the network gets larger. Also, this approach requires that the network topology rarely or never changes during operation and that this topology is known to the database and to most other network elements.
These limitations become a lot more annoying when a network is used for IP data traffic for several reasons. The number of IP users per unit area is usually larger than the number of voice users per unit area. Due to the larger bit rates for IP communication, the cell sizes have to be decreased (becoming microcells and picocells), and thus location changes become much more frequent. Traffic often consists of short bursts instead of long continuous transmissions, and thus the amount of paging relative to the amount of traffic increases. As do operators of data networks in general, operators of cellular networks carrying packet data will likely require their cellular system to be easy to install and to extend, to be failure-proof, robust, and simple.
Another problem area in cellular access networks is that traffic destined for a mobile user must be sent to the user""s current location, which can change even while the user is engaged in a conversation. It is important to minimize the probability and duration of periods when data cannot reach the mobile because its location has become xe2x80x9cunknownxe2x80x9d, but at the same time it is important to avoid sending data to too many locations for safety since that results in a waste of resources.
Most existing cellular systems are connection-oriented, meaning that when data is sent to a remote terminal, a logical channel is established from the point where data enters the cellular network to the base station that the remote terminal is in contact with. This logical channel is used until the mobile terminal leaves the coverage area of the base station (or the conversation is terminated), and the channel is then released. If the mobile terminal moves into the coverage area of another base station, a new logical channel is established, and traffic is switched from the old channel to the new one.
The connection-oriented handling of traffic requires that all the nodes involved in a data path be informed when the connection is established and when it is re-routed or released. For high rates of connection handoffs, this results in a large load of control messages in the network, especially for networks having decreased cell sizes due to increased data rates.
Furthermore, connection-oriented systems tend to have low resource efficiency if traffic is bursty, which is typical of IP traffic. Thus while today""s cellular networks fit voice applications well, they are inefficient for data traffic.
Also, the connection-oriented approach is based on a xe2x80x9cconnection-establishmentxe2x80x9d phase, usually called call-setup, that precedes the transmission of actual voice traffic and that after the conversation there is a xe2x80x9cconnection-releasexe2x80x9d phase. This is generally not the case for IP traffic. The traditional cellular approach is therefore hardly usable for IP traffic because the network does not get an establishment message before user data is sent, so there is no time to set up a connection. Also, an IP network is not informed when a communication is over, and hence an established connection would remain open unless a special mechanism watched user data activity and cleared the connection after a long pause. This obviously results in further problems.
Another problem area of some conventional cellular systems, particularly systems using code division multiple access (CDMA), is that data sent from/to a remote terminal can be sent through multiple base stations, and these duplicates travel in the wired access network separately to/from a gateway to a larger network, such as the Internet. This redundancy (also called macrodiversity) is needed because the air interface may have a large bit error rate so the probability of data packets getting spoiled in the air interface is not negligible. Out of duplicates sent in a macrodiversity state, it is possible to select one that was not spoiled based on, for instance, an estimate of the air interface error rate sent together with each packet.
For a seamless handoff of a data stream or session from one base station or sector to another base station or sector, it is advantageous to establish the data path with the new base station before the one with the old base station is torn down. This form of xe2x80x9cmake-before-breakxe2x80x9d communication is sometimes called xe2x80x9csoftxe2x80x9d handoff, meaning that for a certain time two (or more) data streams are active in parallel. In order to prepare for eventual handoffs, it may be required to keep parallel paths active for a large proportion of operation time.
Unlike in today""s cellular systems, in a cellular system based on IP technology, transmission of packets in the wired network may not be guaranteed. Packets may get lost due to congestion of the wired network or due to hardware failure, and hence in IP systems macrodiversity protects not only against errors in the air interface but also against loss in the wired network. It can be noted that it may be technically feasible to build an IP-based cellular network that guarantees packet transmission in the wired network, but making a cellular network xe2x80x9cbetterxe2x80x9d than the global Internet might be considered wasteful
Macrodiversity requires that data paths be duplicated at some point and re-united at another point. The remote terminal can be responsible for duplicating paths in the uplink (remote-to-base) and re-uniting them in the downlink (base-to-remote). Duplicating in the downlink and re-uniting in the uplink must be performed by devices in the wired access network.
In existing and in most planned systems with macrodiversity, nodes at one level of the hierarchically built cellular network are equipped with macrodiversity combiner devices that combine duplicate information in the uplink and create duplicates in the downlink. These combiner devices are assigned to connections at connection establishment (call setup), and they serve a session until it is released. This solution has at least the following drawbacks.
A combiner device assigned to a path at call setup usually cannot be replaced by another combiner device during the call. Thus, even when a mobile terminal has moved far away from its original location, the two or more data paths have to go separately back to the macrodiversity combiner that was originally selected, increasing network load in vain.
Since a macrodiversity combiner is an expressive device, how many of such devices to place at a node is a serious dimensioning problem. In traditional approaches, the number of devices in a node must be sufficient for all connections that may ever go through the node because there are no devices at multiple network levels and no connection can operate without a macrodiversity combiner. Thus, combiner devices can be poorly utilized, wasting computing resources.
The location of macrodiversity combiners is a trade-off From a dimensioning point of view, combiners should be placed high in the hierarchical access network so that they are as concentrated as possible because concentration always results in higher utilization when there are statistically shared resources. Moreover, the higher is the combination point of duplicate paths, the lesser is the cross-traffic that results from mobile terminals roaming far during active calls. On the other hand, placing combiners high in the hierarchical network makes the length of the duplicate paths long, wasting resources.
Perhaps most important is that the solution used today relies on the connection-oriented nature of cellular traffic. Macrodiversity combiners are assigned to a connection at call setup and are released at call teardown. They cannot be used in a network where the notion of xe2x80x9cconnectionsxe2x80x9d does not exist, as is the case for IP traffic. Data sent by a user using IP is not preceded by a connection-establishment phase and is not followed by a connection-release phase, and it is therefore not possible to allocate macrodiversity devices to a data stream when it starts and release them when it ends.
Applicant""s invention provides a way to build and control wireless networks that carry packet data traffic and a lightweight but robust protocol that enables communication of regular packets, e.g., IP-based packets, to/from a wireless user, such as a mobile terminal in a cellular radio network. The similarity of Applicant""s communication protocol to the conventional Internet protocol permits re-use of IP routers in a cellular radio network, although such re-use is not required. The functionality needed in the nodes of a network in accordance with the invention includes just a subset of the functionality of IP routers, allowing Applicant""s network to use simpler and cheaper devices. The simplicity of the nodes in Applicant""s network is currently believed to be a significant advantage of Applicant""s invention. After all, nodes that are IP routers are currently difficult to use in networks that have space or cost limitations, such as an indoor network having a base station in each room of a structure, since IP routers are complex and expensive.
Applicant""s invention provides the further significant benefits of network scalability and fault tolerance that derive from the distributed nature of the invention. The absence of a central controller makes network installation and extension simple and fast.
One area in which Applicant""s invention can be applied is wireless local area networks (LANs), even if the LAN users were distributed on several floors or in several buildings. Users of portable computers would be provided with a common environment while they move around in the entire office area.
Another important application is providing Internet access to users of cellular mobile telephones. Most existing cellular telephony networks are not optimal for carrying IP traffic, but their infrastructures can readily support a system implemented according to Applicant""s invention. A cellular operator can then appear as an Internet Service Provider for its subscribers.
Although Applicant""s invention may find its common applications in wireless systems, it is believed Applicant""s invention is also applicable in wired environments. In a campus network or in an office building, one may want to create a LAN that is wired but provides terminal mobility. The xe2x80x9cmobile terminalsxe2x80x9d would then connect to xe2x80x9cbase stationsxe2x80x9d via wired connections (using the Ethernet protocol, for example), but they would still have mobility in the sense that they could be connected at any point of the LAN and would have the same IP address without requiring any re-configuration.
The embodiments described here are optimized for packet data, for example IP traffic, but it will be understood that Applicant""s invention can be embodied in networks that handle voice traffic. To ensure voice quality, either an overdimensioned access network or resource allocation methods need to be used. As a packet-based system is currently considerably cheaper in terms of hardware and maintenance than a traditional cellular voice system, an overdimensioned network is still probably commercially competitive.
In accordance with Applicant""s invention, communication networks and network nodes for carrying packet data traffic have architectures that give large freedom in network topology and simplify network extensions, accompanied by a lightweight but robust protocol. A network may be a cellular network that handles packet data traffic and voice traffic. Location information relating to terminals in a network may be determined through terminal-originated packets or control messaging. Stored location information may be cleared by elapse of time or specific messaging. Location information for different kinds of terminals may be stored in a centralized or distributed database.
In one aspect of Applicant""s invention, a communication network for exchanging data packets with a wireline network includes a plurality of nodes, at least one remote terminal for communicating with a node, and a gateway node in communication with the wireline communication network and with the nodes. At least one node includes a database for storing location information, which may have limited lifetime or be updated by specific messages, and each node includes means for forwarding data packets arriving at the node.
Remote terminals may periodically send control packets to the nodes for updating respective location information relating to the remote terminals in the database, which includes a connection management database and/or a location management database for storing location information relating to remote terminals. A connection management database typically stores location information relating to remote terminals that are exchanging data packets with the wireline network. A location management database stores location information relating at least to remote terminals that are not exchanging data packets with the wireline network, and may store location information relating to remote terminals that are exchanging data packets with the wireline network.
Each node includes a predetermined root-port for sending data packets to the gateway node and at least one of either a leaf-port for receiving data packets from a root-port of another node or a port for exchanging packets with remote terminals. Data packets sent from the wireline communication network are directed to the remote terminals based on the location information in a connection management database.
In further aspects of the invention, the control packets may include show-up packets, keep-alive packets, and their opposites. The control packets may be routed from root-port to root-port toward the gateway node, with the show-up packets updating location information in the connection management database(s), the keep-alive packets updating location information in the location management database(s), and their opposites removing location information from the respective databases. Database entries relating to respective remote terminals may each include remote terminal identification information and a list of at least one port through which a packet has arrived from the respective remote terminal within a predetermined time period.
In yet further aspects of the invention, macrodiversity combiner devices may be included in one or more nodes for detecting duplicate data packets arrived at the nodes and for discarding duplicate arrived data packets.
In still further aspects of the invention, a node in a communication network for exchanging data packets with a wireline network has features comparable to those of networks described above.